1. Field of the Invention
The present invention relates to a method for predicting a NOx amount and an exhaust system using the same. More particularly, the present invention relates to a method for precisely predicting the NOx amount contained in an exhaust gas generated at an engine and to an exhaust system controlling supply of a reducing agent or combustion atmosphere according to the NOx amount.
2. Description of Related Art
Generally, exhaust gas flowing out through an exhaust manifold from an engine is driven into a catalytic converter mounted at an exhaust pipe and is purified therein. After that, the noise of the exhaust gas is decreased while passing through a muffler and then the exhaust gas is emitted into the air through a tail pipe. The catalytic converter purifies pollutants contained in the exhaust gas. In addition, a particulate filter for trapping particulate matters (PM) contained in the exhaust gas is mounted in the exhaust pipe.
A denitrification catalyst (DE-NOx catalyst) is one type of such a catalytic converter and purifies nitrogen oxide (NOx) contained in the exhaust gas. If reducing agents such as urea, ammonia, carbon monoxide, and hydrocarbon (HC) are supplied to the exhaust gas, the nitrogen oxide contained in the exhaust gas is reduced in the DE-NOx catalyst through oxidation-reduction reaction with the reducing agents.
Recently, a lean NOx trap (LNT) catalyst is used as such a DE-NOx catalyst. The LNT catalyst absorbs the nitrogen oxide contained in the exhaust gas when the engine operates in a lean atmosphere, and releases the absorbed nitrogen oxide when the engine operates in a rich atmosphere.
In addition, a selective catalytic reduction (SCR) catalyst may be used as the denitrification catalyst. In the SCR catalyst, the reducing agents such as carbon monoxide and total hydrocarbon (THC) react better with nitrogen oxide than oxygen. Thus, such a catalytic converter is called a selective catalytic reduction catalyst.
In a case that such the denitrification catalyst is used, an amount of the reducing agents supplied to the exhaust gas is determined according to a NOx amount contained in the exhaust gas generated at the engine. Therefore, it is very important to predict the NOx amount contained in the exhaust gas in order to improve purification efficiency.
A conventional method for predicting the NOx amount contained in the exhaust gas is to use a map in which the NOx amount generated at each driving condition of the engine is stored. That is, the NOx amount generated at each driving condition is calculated from the map at each time, and a total NOx amount contained in the exhaust gas is predicted by integrating the NOx amount at each time. Since the map, however, is made at a steady state of the engine, the NOx amount predicted based on the map is not correct at a transient state where the driving condition of the engine changes at each time. Particularly, since the engine operates at the transient state longer than at the steady state, a difference between an actual NOx amount and the predicted NOx amount is very large.
Another conventional method for predicting the NOx amount contained in the exhaust gas is to use a NOx sensor mounted at the exhaust pipe upstream of the denitrification catalyst. That is, the NOx amount is detected by the NOx sensor at each time, and the total NOx amount is predicted by integrating the NOx amount at each time. However, the NOx sensor operates normally only when a temperature of the exhaust gas is higher than a predetermined temperature. So, the NOx amount generated until the NOx sensor operates normally cannot be detected. Even if the NOx sensor is used, an error between the actual NOx amount and the detected NOx amount is very large.
The information disclosed in this Background of the Invention section is only for enhancement of understanding of the general background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art already known to a person skilled in the art.